Polyimide films are heat resistant, insulative, solvent resistant, and low-temperature resistant, and it is for this reason that polyimide films have been widely used as a material of electronic and electrical components of computers and IC controls, for example, such as flexible printed circuit boards, base films of TAB carrier tapes, electronic cable coverings for air craft and the like, base films of magnetic recording tapes, and wire rod coverings for superconductive coils. Various types of polyimide films are suitably selected depending on their use.
Therefore, there has been increasing demand for polyimide films and there is a present need to develop a producing process for producing polyimide films with higher productivity.
It has also become common over the last years to use polyimide films in small general devices such as portable phones. The smaller and thinner electronic and electrical components have caused the wiring of the circuits to fine. This change in dimension of parts used in these components may cause the circuit structure of fine wiring to malfunction by wire breakage or shorting, etc. Therefore, those parts used in such electronic and electrical components are required to have highly accurate dimensional stability.
Incidentally, a common producing process of a polyimide film involves casting or coating an organic solvent solution of polyamic acid, which is the precursor, onto a support, followed by solidification and heat treatment. The polyimide film produced in this manner and its producing process had the following problems.
In the foregoing producing process, the process employs either thermal curing or chemical curing. In the case of thermal curing, a solvent is removed from the polyamic acid varnish, which is the polyimide precursor, to form a polyamic acid film, and the polyamic acid film is then converted to a polyimide film by heating. However, in this process, when heating time is reduced, the film fails to show sufficient levels of properties or the film may crack. In chemical curing, a polyamic acid varnish is mixed with a chemical imidizing agent to obtain a gel film, which is cured and dried to obtain the product polyimide film. However, when the polyamic acid film (gel film) which is partially cured and/or partially dried is to be prepared in a shorter period of time to improve productivity, the chemical imidization of the gel film becomes insufficient and as a result basic mechanical strengths of the product polyimide film, such as tear propagation strength, tensile strength, and adhesion strength suffer.
Common procedures of producing the polyimide film proceed as follows. As shown in FIG. 4, a polyamic acid solution composition, which is the polyimide precursor, is mixed with a chemical imidizing agent in an extruder 102. The mixture is spread in a direction of width by the extruder 102 and continuously extruded through a narrow slit opening of a slit die 104 onto an endless belt, on which the mixture forms a flat thin film. The film is imidized while it is dried and cooled to solidify to the extent where the film becomes self-supporting. The film is then subjected to a heat treatment.
Where the polyamic acid composition as the polyimide precursor is used to form the polyimide film by casting using a T die, which involves casting, heating, and drying of the film for the completion of imidization, a sudden onset of the imidization reaction in the process of casting may cause a resin film to partially undergo imidization. This might cause gel defects on the film or the problem of coating stripe which is caused by clogging of the slit die by a partially imidized gel product. While these problems can be effectively solved by controlling the imidization reaction by cooling the polyamic acid solution composition to 0° C. or below; it tends to increase the viscosity, in particular, of the polyamic acid solution composition.
With such a viscosity range, i.e., with the use of a resin solution composition with such a relatively high viscosity, the resin solution composition becomes resilient. In this case, as shown in FIG. 5, a curtain 122 of the fluidic resin solution composition extruded from the slit die 120 is pulled in the machine direction as the speed of the belt becomes faster. Pulling of the curtain 122 in the machine direction makes the landing sheet angle θ between the curtain 122 and the belt 124 of a reel smaller, which may cause the curtain 122 to trap surrounding air when it lands on a surface of the belt 124.
As a result, air is sealed between a surface of the resin film 126 and the belt 124 to leave large and small bubbles of protrusions on the surface of the resin film 126. This air trapping phenomenon has a detrimental effect on the surface of the resin film in the drying step of the resin film, as it causes the resin film to thin or breaks and fluctuates the resin film by expansion of the trapped air.
Further, the high viscosity curtain, because it is more elastic than the curtain of a lower viscosity and has stronger adhesion for the belt, is pulled in the machine direction by the movement of the belt. The curtain pulled by the belt to move over a certain distance in the machine direction is opposed by the force of the opposite direction exerted by the elasticity of the resin film. This opposing force periodically changes the landing site of the curtain, which in turn changes the thickness of the product resin film, with the result that the thickness periodically becomes uneven in the machine direction. Such an uneven thickness appears as a striped pattern on the product film.
As a counter-measure to this problem, Japanese Publication for Unexamined Patent Application No. 198157/1999 (Tokukaihei 11-198157; published on Jul. 27, 1999) discloses a film producing method by casting, in which the viscosity in a die is lowered to prevent air trapping during casting of the resin film and to improve uneven thickness. A lower viscosity in the die is attained by lowering a degree of polymerization of the resin solution composition or by increasing the solvent proportion of the resin solution composition.
However, the mechanical properties of the polyimide film obtained by the method of lowering the degree of polymerization as disclosed in the foregoing publication 11-198157 are significantly poorer than those of the polyimide film obtained from equimolar amounts of diamine component and tetracarboxylic dianhydride component. Further, in the method in which a solvent proportion of the resin solution composition is increased as disclosed in the foregoing publication 11-198157, the temperature of the belt needs to be increased by a large margin to dry the film on the endless belt until the film becomes self-supporting. As a result, the product polyimide film has poor mechanical properties.
As described, in the film producing method by casting as disclosed in the foregoing publication 11-198157 in which air trapping during casting of the resin film is prevented to improve evenness of the film, the mechanical properties of the product polyimide film are considerably poor. Such poor mechanical properties prevent stable production of flexible printed circuit boards, base films of TAB carrier tapes, electronic cable coverings of air craft and the like, base films of magnetic recording tapes, and wire rod coverings for super conductive coils and the like, because the film stretches to generate a slack during their production. The products, as a result, have poor mechanical resistance and poor reliability.
In the foregoing step of solidifying the film before the heat treatment until the film becomes self-supporting, the heat treatment often involves grasping end portions of the film using clips or pins (known as a tenter frame method).
However, in this case, curing of the film cannot be carried out evenly in the transverse direction, and particularly the end portions cannot be cured sufficiently. This is because a grasping jig such as clips or pins prevents a temperature increase at the end portions of the film, or high temperatures of the heat treatment in a heating furnace become uneven when the width of the product polyimide film is wide. Attempts to compensate for the insufficiently cured end portions have resulted in over curing of the central portion, which degrades properties.
The tenter frame method, while it is a suitable conventional technique to maintain or stretch the width of the gel film against cure shrinkage of the gel film in the drying and curing step of the heat treatment in the heating furnace, the held end portions and the unheld central portion often shrink differently. Thus, for the last many years, a solution has been sought for a phenomenon in which the molecular chains of polyimide are oriented in an oblique direction by a 45° angle particularly at the end portions. This anisotropy of molecular orientation is closely associated with properties which relate to dimensional stability, and therefore causes a direction-dependent difference of properties. Such a molecular orientation therefore fails to meet the demand for a material of a flexible printed circuit board and the like of ever increasing precision.
Methods for obtaining isotropic films are disclosed in Japanese Publication for Unexamined Patent Application Nos. 190314/1985 (Tokukaisho 60-190314; published on Sep. 27, 1985), 237928/1993 (Tokukaihei 5-237928; published on Sep. 17, 1993), and 81571/1996 (Tokukaihei 8-81571; published Mar. 26, 1996).
Commonly, a mother roll of the product film is suitably provided with a slit of a predetermined width. It has also become common to produce a wide film so that more products could be made from a single mother roll to increase yield.
Dimensional stability is one property that is required for electronic and electrical components. It is well-known that tensile modulus, which is one of the important parameters of dimensional stability, can be improved with use of monomers having a rigid structure, namely, diamines with high linearity such as paraphenylenediamine, for the diamine component. For example, Japanese Publication for Unexamined Patent Application No. 13242/1989 (Tokukaisho 64-13242; published on Jan. 18, 1989) discloses a three-component polyimide of pyromellitic anhydride, 4,4′-diaminodiphenylether, and paraphenylenediamine. However, a large amount use of rigid and highly linear monomers causes too low coefficient of thermal expansion to be applicable to the laminates with a metal foil like a copper foil. Further, generally, the use of rigid and highly monomers lowers flexibility of the film to cause a problem in bendability which is one of the advantages of the flexible printed circuit board. Further, in order to improve tensile modulus, Japanese Publication for Unexamined Patent Application No. 111359/1986 (Tokukaisho 61-111359; published on May 29, 1986) discloses a four-component polyimide which contains 3,3′-4,4′-biphenyltetracarboxylic dianhydride. However, this technique poses the problem of productivity because it increases the number of monomer components and complicates the polymerization step of polyamic acid, which is the precursor of polyimide. Further, since the technique uses a special type of monomer, it is disadvantageous in terms of cost. Further, Japanese Publication for Unexamined Patent Application No. 20238/1989 (Tokukaisho 64-20238; published on Jan. 24, 1989) discloses improving properties by stretching. However, this technique introduces a complex stretching device in the production process and has a problem that, depending on the type of polyimide, the film may be broken during the stretching process.